The present invention relates generally to a Hall Effect device, and, in particular, to a modified Hall effect device which is a hybrid combination of a first conventional Hall plate and a second conventional Hall plate coupled to a ferromagnetic layer.
Several different types of devices are available for the measurement of magnetic fields. Examples that operate at room temperature include flux gate magnetometers and magnetoresistors. A third family of devices is one of the oldest commercial field sensors, the Hall plate. All of these devices share a common trait; they measure a scalar value of magnetic field that represents a single component of the vector field to be measured. The family of Hall devices includes dozens, if not hundreds, of variations, for example, R. S. Popovic, xe2x80x9cHall-effect Devices,xe2x80x9d Sens. Actuators 17, 39 (1989); and R. S. Popovic, xe2x80x9cHall Effect Devicesxe2x80x9d, (Adam Hilger, Bristol, 1991), both herein incorporated by reference. One limitation of all of these Hall devices is that these devices only can measure a scalar field value proportional to the component of vector field that is perpendicular to the plane of the Hall plate.
In the art, one-dimensional arrays of Hall devices have been used to achieve a moderate magnetic field spatial resolution. A one-dimensional array of Hall crosses has been described by E. Zeldov et al., xe2x80x9cThermodynamic observation of first-order vortex-lattice melting transition in BiSrCaCuO,xe2x80x9d Nature 375, 373 (1995), herein incorporated by reference. In this study, an array of ten Hall crosses, with dimensions of 3 xcexcm by 3 xcexcm for each sensor, was used to detect the motion of fluxons in a small sample of single crystal superconducting BiSrCaCuO. The sensors were fabricated on a GaAs/AlGaAs chip and a macroscopic (0.7 mm by 0.3 mm by 0.1 mm) sample was placed on top. Each Hall sensor had a DC sensitivity of order 0.1 Oe and each was able to sense the presence of a single fluxon when in proximity to the sensor. The spatial resolution of the measurement was therefore of the order of magnitude of the Hall sensor dimensions, a few microns.
One recent advancement in the art of measuring magnetic fields is provided by a hybrid Hall effect device. A hybrid Hall device is a simple bilayer magnetoelectronic device. The physical principles of operation are described in the publication xe2x80x9cHybrid Hall Effect Device,xe2x80x9d by Mark Johnson, B. Bennett, M. J. Yang, M. M. Miller and B. V. Shanabrook, Appl. Phys. Lett. 71, (1997) and in U.S. Pat. No. 5,652,445 entitled xe2x80x9cHybrid Hall Effect Device and Method of Operating,xe2x80x9d both herein incorporated by reference, and are briefly described below with reference to FIGS. 1(a) and (b).
An example of one prior art hybrid Hall effect device is generally denoted 10 in FIGS. 1(a) and 1(b). A thin, microstructured ferromagnetic film F, denoted 11, is fabricated over a standard Hall cross 12 formed from a Hall plate 14, and positioned such that edge 13 of film 11 is disposed over the central region of the Hall cross 12. The film 11 is electrically isolated from the Hall cross 12, typically by a thin insulating layer 15.
The film 11 has a magnetization anisotropy in the film plane and acts as a local source of magnetic field. When an external magnetic field Hx{circumflex over (x)} causes the magnetization M of F to be along xe2x88x92{circumflex over (x)}, there is a positive magnetic pole density on the edge over the Hall cross 12, and a local negative field xe2x88x92|Bz| is generated in the vicinity of the carriers in the Hall plate 14 which comprises an InAs layer 16, a second insulating layer 17 and a substrate 18 (see FIG. 1(b)). The result is a positive sense voltage Vs=V+xe2x88x92Vxe2x88x92. If the magnetization orientation is reversed, the sign of the pole density, the local field |Bz|, and the sense voltage V; is reversed. Thus, device 10 translates a horizontal magnetic field into a vertical magnetic field, thereby providing for the detection and measurement of the convolution of the {circumflex over (x)} and {circumflex over (z)} field components comprising the magnetic field.
When fabricated with appropriate magnetic properties, the film 11 has bistable magnetization and the resulting hybrid Hall effect device 10 has digital applications, such as nonvolatile storage. When fabricated with other magnetic properties, the magnetization of film 11 responds linearly with an external field. A hybrid Hall effect device engineered in this way can act as a sensor of in-plane magnetic fields.
One limitation with prior art hybrid Hall effect devices, such as device 10, is that these prior art devices cannot be used to determine the independent magnetic field vector components comprising a vector magnetic field, such as for determining the {circumflex over (x)} and the {circumflex over (z)} components of a magnetic field; device 10 merely measures the convolution of the {circumflex over (x)} and {circumflex over (z)} field components.
The present invention concerns a modified hybrid Hall effect device, which is a hybrid combination of a first conventional Hall plate and a second conventional Hall plate coupled to a ferromagnetic layer, corresponding to film F. The modified Hall Effect device can be adapted for use as a magnetic field sensor capable of measuring the individual components of a vector magnetic field. Equivalently, the device is capable of measuring the magnitude of a vector magnetic field of any orientation.
In the present modified hybrid Hall effect device, the ferromagnetic film acts as a field transducer that translates a horizontal field into a vertical field. This is a function that can be combined with the sensitivity of Hall devices for use in applications where perpendicular and horizontal magnetic field components are present, to make a magnetometer that is sensitive to more than a single component of the field.
An object of the present invention is to provide an inexpensive, integrated combination of a Hall device that is adaptable for measuring two or three components of a vector magnetic field, {overscore (B)}. A magnetometor, incorporating the present invention, which is capable of measuring two or more vector components of a magnetic field, has several advantages over magnetometers which are capable only of scalar measurements. The single component to which a scalar magnetometer is sensitive may be the smallest component of the vector field to be measured. Therefore, measuring all three vector components, the vector magnetometer always measures the total field magnitude, and is therefore intrinsically more sensitive in any application.
An additional object of the present invention is to provide a microfabricated magnetometer cell with two or three components for measuring a vector field with high spatial resolution. A specific application is the calibration and characterization of magnetic force microscope (MFM) tips. A cell that measures a single component of field can also be used for this application, with diminished field sensitivity by comparison with the two or three component cells.
A further object of the present invention is to provide a device, which is adaptable to use any source of magnetic field and which generates a three-dimensional magnetic field that is dipolar but has properties unique to that source. The source may be either a macroscopic source, such as a vehicle, or a microscopic source such as a magnetic particle used to tag a molecule or bundle of molecules. While measuring a scalar field might permit the detection of a source of the field, measuring a vector field permits the detection of a source and, with a suitable model, also permits the identification of the object that is the source. Following this functionality, measurement of a vector field as a function of time and/or position permits an analysis of position and motion of the object, which is the field source.
An additional object of the present invention is to provide an inexpensive vector magnetometer adaptable for use in remotely sensing large objects that have intrinsic magnetic moments.
Yet another object of the present invention is to provide a microfabricated vector magnetometer with high spatial resolution that can be adapted to scan across a magnetic field that includes an object of interest.
A further object of the present invention is to provide novel techniques for forming two-dimensional arrays of Hall cells, and new applications employing arrays of Hall devices for sensing magnetic fields with high spatial resolution.
Another object of the present invention is to provide a two-dimensional array of sensor cells for high spatial resolution of objects that have local magnetic fields that vary on a small spatial scale.
Yet a further object of the present invention is to provide a sensor cell that can be adapted for use as a susceptometer, to detect one, two or three components of magnetic field when a perturbing magnetic field is applied to an object.
A further object of the present invention is to provide susceptometers for use in applications where an external perturbing magnetic field is applied to the object that is the source of magnetic field to be measured, even if the perturbing field is also applied to the sensor.
In accordance with the aforementioned objects, the present invention concerns an integrated Hall effect device for measuring the vector components of an external magnetic field. The device includes a conductive substrate having a top surface with a first section and a second section and is capable of carrying an electrical current. The first section is positioned in close proximity to the second section such that a first electrical signal can be generated in response to one of the vector components of the external magnetic field acting perpendicularly on the electrical current in the conductive substrate. A ferromagnetic film is provided having at least one configurable magnetization orientation state and the ferromagnetic film is disposed over a portion of the second section such that a fringe magnetic field can be generated from an edge of the ferromagnetic film by vector components of the external magnetic field acting on the magnetic film wherein a second electrical signal can be generated in response to the fringe magnetic field acting on an external current in the conductor substrate. In one further specific form, the conductive substrate and the ferromagnetic film are separated by an insulating layer.
According to another aspect of the present invention, a method is provided for calibrating a magnetic force microscope (MFM) tip by measuring vector components of a magnetic field generated by the microscope tip. The method includes the steps of generating an electrical current that flows along a current axis of a conductor substrate, scanning the microscope tip across the conductor substrate at a variable height, and generating a first electrical signal along a first voltage axis in response to a vector component of a magnetic field from the microscope tip acting on the electrical current in the conductor substrate. The values of the first electrical signal are mapped as a function of the variable height.
In one specific further embodiment, the microscope tip is scanned across a ferromagnetic film disposed over a portion of the second section at a variable height and a corresponding fringe field is generated at an edge of the ferromagnetic film in response to vector components of a magnetic field from the microscope tip acting on the ferromagnetic film. The edge of the ferromagnetic film is located adjacent to a second voltage axis such that the corresponding fringe field is perpendicular to the current axis. A second electrical field is generated along the second voltage axis in response to the fringe field acting on the electrical current in the connected substrate. The values of the first electrical signal and the second electrical signal are mapped as a function of the variable height.
In accordance with another aspect of the present invention, a method is provided for identifying chemical or biological agents that have been tagged with microscopic magnetic particles by measuring vector components of a magnetic field generated by the magnetic particles. The method includes generating an electrical current that flows along a current axis of a conducting substrate mounted on a proximal probe tip. The substrate is scanned for chemical or biological agents and a first electrical signal along a first voltage axis is generated in response to vector components of a particle""s magnetic field acting on the electrical current in the conductive substrate. The first electrical signal is recorded and values of the first electrical signal are mapped with respect to magnetic fields associated with the magnetic particles.
In one further specific embodiment, the chemical or biological agents are scanned by a vector magnetometer mounted on a proximal probe tip and a fringe field is generated at an edge of a ferromagnetic film in response to a vector component of a particle""s magnetic field acting on the ferromagnetic film where the edge of the ferromagnetic film is located adjacent to a second voltage axis such that the fringe field is perpendicular to the current axis. A second electrical signal is generated along the second voltage axis in response to the fringe field acting on the electrical current in the second section of the conducting substrate. The second electrical signal is recorded and values of the second electrical signal, along with the values of the first electrical signal, are mapped with respect to magnetic fields associated with the magnetic particles.
In accordance with yet another aspect of the present invention, a cell array is provided which includes integrated Hall effect devices for measuring the vector components of an external magnetic field of an object. The cell array includes a plurality of rows comprising a plurality of integrated Hall effect devices. Each Hall effect device is electrically connected to each other along a current axis. Each of the plurality of integrated Hall effect devices includes a conductor substrate having a top surface with a first section and second section, which are capable of carrying an electrical current. The first section is positioned in close proximity to the second section such that a first electrical signal can be generated in response to one of the vector components of the external magnetic field acting perpendicularly on the electrical current in the conducting substrate. A ferromagnetic film is provided having at least two configurable magnetization orientation states and is positioned disposed over a portion of the second section such that a fringe magnetic field can be generated from an edge of the ferromagnetic film by vector components of the external magnetic field acting on the magnetic film wherein a second electrical current can be generated in response to the fringe magnetic field acting on an electrical current in the conductor substrate.
The present invention offers numerous advantages over prior devices. For example, sensors based on Hall devices according to the present invention offer numerous advantages and are superior in many ways to MR sensors known in the art. The present sensors have linear response over a large field range, and they can be fabricated on a small size scale with highly reproducible characteristics.
In addition, the present invention provides for a superior magnetometer that is sensitive to single components of field. In addition, a vector magnetometer, in accordance with the present invention, can be adapted to sense a magnetic field and, using knowledge and properties of the vector components, can also be used to detect and identify an object that is the source of field. Further, the present vector magnetometer may also be used to analyze the position and motion of such an object.
A further advantage of the present invention is that the present device is readily integrated and can be manufactured and packaged using inexpensive techniques. It is therefore inexpensive, and is much less expensive than scalar magnetometers such as flux gates and SQUIDs.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.